matter so much, but for older samples it can be a serious problem because so few remaining atoms are being counted. In the first instance, to borrow from Flannery, it is like miscounting by a dollar when counting to a thousand; in the second it is more like miscounting by a dollar when you have only two dollars to count.
Libby’s method was also based on the assumption that the amount of carbon-14 in the atmosphere, and the rate at which it has been absorbed by living things, has been consistent throughout history. In fact it hasn’t been. We now know that the volume of atmospheric carbon-14 varies depending on how well or not Earth’s magnetism is deflecting cosmic rays, and that that can vary significantly over time. This means that some carbon- 14 dates are more dubious than others. This is particularly so with dates just around the time that people first came to the Americas, which is one of the reasons the matter is so perennially in dispute.
Finally, and perhaps a little unexpectedly, readings can be thrown out by seemingly unrelated external factors-such as the diets of those whose bones are being tested. One recent case involved the long-running debate over whether syphilis originated in the New World or the Old. Archeologists in Hull, in the north of England, found that monks in a monastery graveyard had suffered from syphilis, but the initial conclusion that the monks had done so before Columbus’s voyage was cast into doubt by the realization that they had eaten a lot of fish, which could make their bones appear to be older than in fact they were. The monks may well have had syphilis, but how it got to them, and when, remain tantalizingly unresolved.
Because of the accumulated shortcomings of carbon-14, scientists devised other methods of dating ancient materials, among them thermoluminesence, which measures electrons trapped in clays, and electron spin resonance, which involves bombarding a sample with electromagnetic waves and measuring the vibrations of the electrons. But even the best of these could not date anything older than about 200,000 years, and they couldn’t date inorganic materials like rocks at all, which is of course what you need if you wish to determine the age of your planet.
The problems of dating rocks were such that at one point almost everyone in the world had given up on them. Had it not been for a determined English professor named Arthur Holmes, the quest might well have fallen into abeyance altogether.
Holmes was heroic as much for the obstacles he overcame as for the results he achieved. By the 1920s, when Holmes was in the prime of his career, geology had slipped out of fashion-physics was the new excitement of the age-and had become severely underfunded, particularly in Britain, its spiritual birthplace. At Durham University, Holmes was for many years the entire geology department. Often he had to borrow or patch together equipment in order to pursue his radiometric dating of rocks. At one point, his calculations were effectively held up for a year while he waited for the university to provide him with a simple adding machine. Occasionally, he had to drop out of academic life altogether to earn enough to support his family-for a time he ran a curio shop in Newcastle upon Tyne-and sometimes he could not even afford the ?5 annual membership fee for the Geological Society.
The technique Holmes used in his work was theoretically straightforward and arose directly from the process, first observed by Ernest Rutherford in 1904, in which some atoms decay from one element into another at a rate predictable enough that you can use them as clocks. If you know how long it takes for potassium-40 to become argon-40, and you measure the amounts of each in a sample, you can work out how old a material is. Holmes’s contribution was to measure the decay rate of uranium into lead to calculate the age of rocks, and thus-he hoped-of the Earth.
But there were many technical difficulties to overcome. Holmes also needed-or at least would very much have appreciated-sophisticated gadgetry of a sort that could make very fine measurements from tiny samples, and as we have seen it was all he could do to get a simple adding machine. So it was quite an achievement when in 1946 he was able to announce with some confidence that the Earth was at least three billion years old and possibly rather more. Unfortunately, he now met yet another formidable impediment to acceptance: the conservativeness of his fellow scientists. Although happy to praise his methodology, many maintained that he had found not the age of the Earth but merely the age of the materials from which the Earth had been formed.
It was just at this time that Harrison Brown of the University of Chicago developed a new method for counting lead isotopes in igneous rocks (which is to say those that were created through heating, as opposed to the laying down of sediments). Realizing that the work would be exceedingly tedious, he assigned it to young Clair Patterson as his dissertation project. Famously he promised Patterson that determining the age of the Earth with his new method would be “duck soup.” In fact, it would take years.
Patterson began work on the project in 1948. Compared with Thomas Midgley’s colorful contributions to the march of progress, Patterson’s discovery of the age of the Earth feels more than a touch anticlimactic. For seven years, first at the University of Chicago and then at the California Institute of Technology (where he moved in 1952), he worked in a sterile lab, making very precise measurements of the lead/uranium ratios in carefully selected samples of old rock.
The problem with measuring the age of the Earth was that you needed rocks that were extremely ancient, containing lead- and uranium-bearing crystals that were about as old as the planet itself-anything much younger would obviously give you misleadingly youthful dates-but really ancient rocks are only rarely found on Earth. In the late 1940s no one altogether understood why this should be. Indeed, and rather extraordinarily, we would be well into the space age before anyone could plausibly account for where all the Earth’s old rocks went. (The answer was plate tectonics, which we shall of course get to.) Patterson, meantime, was left to try to make sense of things with very limited materials. Eventually, and ingeniously, it occurred to him that he could circumvent the rock shortage by using rocks from beyond Earth. He turned to meteorites.
The assumption he made-rather a large one, but correct as it turned out-was that many meteorites are essentially leftover building materials from the early days of the solar system, and thus have managed to preserve a more or less pristine interior chemistry. Measure the age of these wandering rocks and you would have the age also (near enough) of the Earth.
As always, however, nothing was quite as straightforward as such a breezy description makes it sound. Meteorites are not abundant and meteoritic samples not especially easy to get hold of. Moreover, Brown’s measurement technique proved finicky in the extreme and needed much refinement. Above all, there was the problem that Patterson’s samples were continuously and unaccountably contaminated with large doses of atmospheric lead whenever they were exposed to air. It was this that eventually led him to create a sterile laboratory-the world’s first, according to at least one account.
It took Patterson seven years of patient work just to assemble suitable samples for final testing. In the spring of 1953 he traveled to the Argonne National Laboratory in Illinois, where he was granted time on a late- model mass spectrograph, a machine capable of detecting and measuring the minute quantities of uranium and lead locked up in ancient crystals. When at last he had his results, Patterson was so excited that he drove straight to his boyhood home in Iowa and had his mother check him into a hospital because he thought he was having a heart attack.
Soon afterward, at a meeting in Wisconsin, Patterson announced a definitive age for the Earth of 4,550 million years (plus or minus 70 million years)-“a figure that stands unchanged 50 years later,” as McGrayne admiringly notes. After two hundred years of trying, the Earth finally had an age.
His main work done, Patterson now turned his attention to the nagging question of all that lead in the atmosphere. He was astounded to find that what little was known about the effects of lead on humans was almost invariably wrong or misleading-and not surprisingly, he discovered, since for forty years every study of lead’s effects had been funded exclusively by manufacturers of lead additives.
In one such study, a doctor who had no specialized training in chemical pathology undertook a five-year program in which volunteers were asked to breathe in or swallow lead in elevated quantities. Then their urine and feces were tested. Unfortunately, as the doctor appears not to have known, lead is not excreted as a waste product. Rather, it accumulates in the bones and blood-that’s what makes it so dangerous-and neither bone nor blood was tested. In consequence, lead was given a clean bill of health.
Patterson quickly established that we had a lot of lead in the atmosphere-still do, in fact, since lead never goes away-and that about 90 percent of it appeared to come from automobile exhaust pipes, but he couldn’t prove it. What he needed was a way to compare lead levels in the atmosphere now with the levels that existed before 1923, when tetraethyl lead was introduced. It occurred to him that ice cores could provide the answer.
It was known that snowfall in places like Greenland accumulates into discrete annual layers (because seasonal temperature differences produce slight changes in coloration from winter to summer). By counting back
